Developmental Biology

• Embryology – traditional term for the study of animal development.    
• Embryo – developing organism (gives rise to multicellular organisms through development.)
• Zygote – fertilized egg; divides mitotically to produce all cells in the body.
• Development is continuous – skin cells are replaced every day, bone marrow produces RBCs every minute; frogs and butterflies undergo metamorphosis.
• Animal Development – characterized by the differentiation of the fertilized egg into the many cell types of the body and by the construction of functionally integrated organs.
• Development is the route via which an organism goes from genotype to phenotype and it can be studied at any level of organization from molecules to ecosystems.
• Fertilization – union of egg and sperm cell in humans
• Cleavage – dividing of cells
• Gastrulation – formation of gastrula
• Organogenesis – formation of organs from a set of cells
• Metamorphosis – changing from one stage to another
• Regeneration – some animals exhibit regeneration of body parts
• Senescence – cell cycle arrest
• Aristotle – first known embryologist; “wonder was the source of knowledge.”
• Our answers to these questions must respect the complexity of the inquiry and must explain a coherent causal network from gene through functional organs.
• Developmental Biology – the discipline that studies embryonic and other developmental processes.
• Two Major Objectives:

  1.     It generates cellular diversity and order within the individual organism.

  2.     It ensures the continuity of life from one generation to the next.
• A single cell, the fertilized egg, gives rise to hundreds of different cell types – muscle cells, epidermal cells, neurons, lens cells, lymphocytes, blood cells, fat cells and so on.
• This generation of cellular diversity is called differentiation.
• Our differentiated cells are not randomly distributed. Rather, they are organized into intricate tissues and organs. During development, cells migrate and die; tissues fold and separate. This creation of an ordered form is called morphogenesis, and it involves coordinating cell growth, cell migration and cell death.
• The sperm and egg are highly specialized cells, and only they can transmit the instructions for making an organism from one generation to the next.
• Some salamanders regenerate their eyes and their legs, while many reptiles can regenerate their tails.
• While mammals are generally poor at regeneration, there are some cells in our bodies – stem cells – that are able to form new structures even in adults.
• The development of many (perhaps all) organisms is influenced by cues from the environment that surrounds the embryo or larva.
• Sex determination – the sex of many species of turtles, for instance, depends on the temperature of the embryo experiences while in the egg. Moreover, certain chemicals in the environment can disrupt normal development, causing malformations in the adult.
• Evolution involves inherited changes of development.
• The study of development has become essential understanding all other areas of biology.

  1.     Molecular Biology

  2.     Physiology

  3.     Cell Biology

  4.     Genetics

  5.     Anatomy

  6.     Cancer Research

  7.     Neurobiology

  8.     Immunobiology

  9.     Ecology

  10. Evolutionary Biology
• Developmental biology studies the building of organisms. It is a science of becoming, a science of process.
• One of the major triumphs of descriptive embryology was the idea of a generalizable animal life cycle. Modern developmental biology investigates the temporal changes of gene expression and anatomical organization along this life cycle.
• The stages of development between fertilization and hatching are collectively called embryogenesis.
• Fertilization – fusion of mature sex cells which are collectively called gametes. The fusion of these gametes stimulated the egg to begin development and initiate a new individual.
• Cleavage – a series of extremely rapid mitotic divisions that immediately follow fertilization. During cleavage, the enormous volume of zygote cytoplasm is divided into numerous smaller cells called blastomeres. By the end of cleavage, the blastomere have usually formed a sphere, known as a blastula.
• Gastrulation – happens after the rate of mitotic divisions that immediately followed have slowed down and blastomeres undergo dramatic movement and changes in position. This is when the embryo is said to be in the gastrula stage. Results to embryo containing three germ layers (endoderm, ectoderm, and mesoderm).
• Organogenesis – once the germ layers are established, the cells interact with one another and rearrange themselves to produce tissues and organs.
• Metamorphosis – organism undergoes change to become sexually mature adult.
• Gametogenesis – development of gametes which is usually not completed until the organisms is physically mature.
• Comparative Developmental Anatomy

  First studied by Aristotle in The Generation of Animals (ca. 350 BCE), he noted some of the variations on the life cycle themes.
• Holoblastic – where the entire egg is divided into successfully smaller cells (frogs and mammals).
• Meroblastic – where only part of the part of the egg is destined to become the embryo, while the other portion known as the yolk serves as nutrition for the embryo.
• 1651: William Harvey concluded that all animals including mammals originated from eggs; Ex ovo omnia